Add more torch.nn modules

[davoclavo]

Implement more modules from https://pytorch.org/docs/stable/nn.html

TODO

• Add implementation for more modules
• Add tests
• Add documentation using the guide™️
• Figure out what to do with Sequence + Embeddings (see Support TensorModule of distinct input/output types #34)
• Break out modules into traits to avoid forgetting an export (akin to ops)
• Evaluate if using given Conversion is a good approach to replace some extension methods

[sbrunk]

@davoclavo are you planning to work on this in the near future?

The reason I'm asking is that I want to implement a few transformer models and I need the Embedding module for that.

If not, I might pull your implementation out from here into its own commit (keeping you as author of course) if that's OK for you.

[davoclavo]

@sbrunk Hi! Thanks for checking. I have implemented some modules already but haven't had time to add documentation and other things. Let me upload what I have and feel free to pick up from there.

[sbrunk]

Awesome, thanks!

[davoclavo]

@sbrunk Done, I just uploaded the changes I had (ModuleList and LayerNorm), feel free to pick up the branch as is and add your changes.

[sbrunk]

Hey @davoclavo I've added tests and examples for the modules and a small fix for LayerNorm actually discovered by a test. :)

I think we should get this merged and tackle the remaining TODOs in later PRs. Would that be ok for you?

[davoclavo]

@sbrunk Sorry for the delay, I was traveling and somehow missed the notification. Thanks for the fixes and the merge! The remaining TODOs can definitely be addressed in future PRs